The Bathtub Displacement
by WeBuiltThePyramids
Summary: Penny's class is cancelled for a day, but she doesn't want to go a day without learning something new. So she enlists the help of her favorite scientist, who has an idea for a lesson. Rated T plus.


**So anyone on the Leonard and Penny thread at Fanforum will be needing absolutely zero backstory or context on this. For those that aren't, basically we came up with some more ways Leonard and Penny can turn each other on with science. This is one of them.**

**I don't own anything. **

Leonard looked surprised when his girlfriend waltzed into the room at this time on a "school day." "Penny," he said, smiling. "What are you doing here?"

"My professor is sick, so class is cancelled," Penny said.

"Oh, that's too bad," Leonard replied, sounding genuinely disappointed that she wouldn't have an opportunity to learn that day. "I'm sorry."

"Don't be," Penny said. "The professor is…eh…"

"You don't like your teacher?" Leonard asked. "Why not?"

"It's not that I don't like him," she said. "It's just that…he's not my favorite teacher."

"You only have one class."

She rolled her eyes, only half annoyed. "Well, the guy who taught me about holograms and relativity has much more going for him than my history professor," she said. "I find his methods to be a bit more…" she walked up to him and smiled. "Effective."

Realization hits him. "_Oh._" He blushed slightly, looking at her sideways. She grinned at his bashfulness, and then it passed and he assumed a more confidant pose. "So, you think you might be wanting to learn something today, since you won't be able to at school?"

"Maaaaybe," she said, grinning now that she knew her point had come across. Some of her old boyfriends would have picked up on her intentions when she waltzed into their apartments wearing shorts in the middle of the winter, but it always took Leonard an extra second to get where she was coming from. Penny found that adorable.

Leonard bit his lip, thinking, and then a slow smile came across his face. "Come with me," he said, grabbing her hand. She put her other hand over his and followed him into the bathroom.

"The science of bathrooms?" she guessed, knowing it was wrong but having no idea what his plan was.

He smiled. "Nope." He knelt down by the tub and turned the water on, jumping when the stream shot out of the shower head, hitting the bottom of the tub near the opposite wall.

"It sticks a bit," Penny said, smirking.

Leonard looked up at her. "Thanks." He stood up, letting the tub fill from the shower head. "Have you ever heard of Archimedes?"

Penny crossed her arms. "Refresh my memory."

"Okay," Leonard said, "Archimedes was a mathematician in Ancient Greece…"

"Oh balls," Penny muttered.

Leonard looked at her. "What?"

"I think Sheldon tried to tell me about Archimedes once," Penny said. "But he's a terrible teacher, go on."

Leonard looked at her. "If you intend to bring Sheldon into this discussion I don't think I'll be able to end this session the way I think you were hoping."

"Sorry, sorry sorry!" Penny said. "Keep going. Archimedes."

"Basically," Leonard said, the king at the time – Hiero II – gave a goldsmith some gold to make a crown out of. He had suspicions that the goldsmith had taken some of the gold out to keep for himself and replaced it with silver, but he didn't know how to prove it, so, he asked Archimedes, the brilliant mind, to figure out a way of proving it. Archimedes…" Leonard paused to turn off the shower – the tub was sufficiently filled to the brim – and turned back to Penny "was getting into a bathtub while thinking about it, and he noticed that as more of him got in the water, the higher the water level rose, and he concluded that the amount of water that spilled out over the tub would be equal to his own volume, and he could measure the volumes of the crown in the water in the same way. This was how displacement was 'discovered'."

"Wow," Penny said. "So…" she kicked off her flip flops and stepped into the tub. "The amount of water that just came over the edge is the volume of how much of me is under the water."

Leonard nodded, smiling. "Exactly!"

"So if I laid down," she said, "then the displaced water would be equal to me?"

"Uh-huh," Leonard said. "Providing nothing else was in the water." He gestured to her discarded flip flops. "If you were standing the water with those on, then the moved water would be the total volume, including the shoes."

Penny nodded. "So if I were to lay down right like this, then it wouldn't be an accurate measurement of my volume." In one swift motion, her shirt was pulled over her head and tossed over by the flip flops. "Because my clothes would mess it all up." She grinned, noticing a similar expression on Leonard's face as when she put on the glasses, or when she held up the Victoria's Secret bag that had prompted him to leave the ring competition. It was his turned on face, she knew it well, and she raised her eyebrows slightly. "Right? Is this better?"

Leonard cocked his head to one side, then the other. "Well…your shorts would have to be included in the margin of error."

She stepped out of the tub and looked down at it. "I wonder what both of our volumes would be," she asked. "Don't you?"

He gave a slow nod. "We should probably figure that out."

Penny took the step that separated them and planted her lips on his, her hands working to rid him of his jacket. By the time it was off and she went for his shirt, he'd unhooked her bra and tossed it. Penny gasped when she heard it land in the bathtub.

"What's wrong?" Leonard asked, looking curiously at her.

"It's going to affect the results!" Penny said, pointing at the underwear.

"Well, we can't be losing credibility, now can we?" Leonard asked reaching over and removing the bra from the water.

"I'll wring it out," Penny said. "You take off your pants."

"Yes ma'am," Leonard said, obliging as she removed as much water as she could from the bra before tossing it with her other clothes.

"Can I ask you something?" Leonard asked her as they removed their last bits of clothing.

"Yeah," she said, tapping him on his chest.

He stepped backward into the tub, and held out a hand for her to join him. "Remember how Sheldon said he'd never heard 'yee haw' used in this context?"

"Remember what you told me about Sheldon?" Penny said. "That applies on this end, too."

"Ah, right, I mean…" Leonard cocked his head. "Could you maybe change that to 'eureka'?"

"Eureka!" Penny said. "Why's that? Sit down."

Leonard lowered himself into the water, leaning back against the wall, and Penny dropped down in front of him. "We're displacing a lot of water," she noted. "But seriously, why 'eureka'?"

"Well," Leonard said, "when Archimedes saw the water sloshing out of the tub, that was when it gave him the solution to his problem. He jumped out of the tub and ran down the street buck naked shouting 'eureka!'"

Penny ran her tongue over her upper lip. "That is _so_ hot."

"Eh…" Leonard said. "It's probably better that you don't know what Archimedes looked like."

"Well I don't, and I think it's hot." She gave him her best seductive smile. "_Displacement_."

He shuddered. "Any science word will do it, huh?" She teased.

Leonard tried to mimic her look. "Displacement, Penny. Displacement."

Her eyes widened. "Holy Hell, that actually works."

"So," Leonard said, tracing the outline of her Cookie Monster tattoo with his pointer finger, "the eureka thing. Will you do it?"

She leaned over and kissed him, running her hands down his chest. "I'll make sure Mrs. Gunderson knows that you taught me all about Archimedes and displacement. You even had a shiny object!"

Leonard grinned, and Penny scooted closer to him, adjusting her position so they could end the learning session the way she had been hoping.

* * *

Sheldon and Amy were crossing the third floor landing when they both jumped at the sound of a shrill female voice. "What was that?" Amy said, looking confused.

"I believe that it was someone saying 'eureka'," Sheldon said.

"It sounded like Penny," Amy said thoughtfully.

Sheldon looked over at her. "That it did. That it did." He frowned. "That's odd, Penny couldn't possibly have learned anything to warrant Archimedes' triumphant cry."

Amy looked thoughtful, and then a slow grin came over her face. "_Nice_."

Sheldon looked at her oddly, and she just kept grinning. Then a horrified look came over his face. "No."


End file.
